For certain electrical devices the ability to control or adjust the level of power supplied to them is advantageous. In these devices what may be generally described as a load power control circuit provides the function of allowing a user to provide this control by adjustment of an element, for example a potentiometer, in the circuit.
There are a number of situations where this need arises. In a particular application of interest, it is desirable to be able to allow manual control of the illumination level provided by fluorescent lighting. In the most recent types of such dimmable fluorescent lighting systems, power is provided to each individual fixture through what is called an electronic ballast which functions in a fluorescent lighting fixture as the aforementioned load power control circuit. In one particular commercial design, the dimming level is adjusted by varying the value of an external variable control impedance which is connected across a pair of the ballast's control terminals. There is, internal to the ballast, a current source in parallel with a resistance across the pair of ballast control terminals. By varying the control impedance across the ballast control terminals a dimming control signal voltage is created across the control terminals which is sensed by other elements of the ballast's internal circuitry and in response to which vary the illumination level provided by the fixture of which the ballast is a part. The control voltage across the control terminals varies from about 1 volt at minimum illumination to about 10 v. at full brightness. Each ballast provides power to a pair of fluorescent bulbs.
It is possible, by ganging the control terminals for the ballasts across the control impedance circuit terminals, to connect a number of individual ballasts' control terminals to a single control impedance circuit. In this commercial design, the control impedance circuit includes active semiconductor elements which make the control characteristics of the impedance circuit as a function of its adjustment potentiometer resistance nearly insensitive to the number of ballasts controlled by the impedance circuit. That is, the illumination level of individual fixtures is very nearly the same for a given mechanical position of the control impedance circuit's adjustable element regardless of the number of ballasts controlled by the impedance.
The control impedance circuit has the capability of controlling the dimming for as many as 60 individual ballasts. The limitation on the number of ballasts which may be controlled by a single control impedance is directly related to the ability of the impedance to sink the current which each individual ballast produces at its control terminals.
In certain applications it is useful to be able to control more than the designed-for number of 60 fixtures from a single impedance. While 60 fixtures at first blush appears to be a large number, many commercial and office buildings have literally hundreds of fluorescent fixtures whose control by a single control element is sometimes desirable. To provide a control impedance with greater capability than the 60 ballasts requires a built-in power supply which increases its production and installation cost. It is desirable to devise some means of avoiding these aforementioned limitations. In particular, a means for transparently interfacing between a single control impedance and a large number of fluorescent fixtures would be very useful.
There are a number of references pertaining to varying the amount of electric power applied to a load. In the particularly pertinent electric lamp dimming control field, U.S. Pat. No. 4,628,230 shows a light dimming circuit for use with a plurality of lamps and which uses a feedback signal in controlling the illumination level. U.S. Pat. Nos. 4,645,979; 4,651,060; 4,686,427; 4,704,563; 4,712,045; and 4,717,863 are other references showing dimming circuits for fluorescent lamps.
A discussion of a particular aspect of the theory of circuit equivalence will be helpful in understanding the invention to be described. The concept of a current source is well known to those skilled in the electronic arts, and indeed, the commercial embodiment of the electronic ballast mentioned above uses a current source in parallel with a resistor as the power source at its input terminals. It is known that one can substitute a current source in parallel with a resistor for a voltage source in series with a resistor of a different value to provide equivalent electrical characteristics. Therefore, for the remainder of this discussion, one should consider a current source in parallel with a resistor of some value to be interchangeable with a voltage source in series connection with a resistor. In particular, use of the term "voltage source" is not meant to limit the disclosure involved to that specific embodiment, and the current source equivalent should be understood to be included in the term.